This invention relates to the preparation of elastomeric organopolysiloxanes. This invention further relates to the preparation of elastomeric, heat-resistant organopolysiloxanes which do not revert to lower molecular weight liquids or semi-solids when exposed to elevated temperatures.
It is well known to prepare elastomeric, polymeric organopolysiloxanes by reacting a precondensed organosiloxane of the general formula EQU XO--Si(R).sub.2 --O-SiR.sub.2 ].sub.n O-Si(R).sub.2 --OX
wherein R is alkyl or aryl, X is hydrogen or alkyl and n represents an integer of 50 or more, with a polyfunctional silicon-containing crosslinking agent such as a tri- or tetraalkoxy silane in the presence of a suitable catalyst. U.S. Pat. No. 3,127,363 discloses that this reaction can be carried out under relatively mild conditions if a filler such as carbon black, one of the various types of clays or diatomaceous earth is present during the reaction of the precondensed organosiloxane with the curing agent. The resultant cured materials are disclosed as being useful for a variety of applications, including sealants, casting compositions, coatings, encapsulants and molding compositions. The aforementioned U.S. Pat. No. 3,127,363 teaches that any of the known catalysts for curing silicone resins can be employed. These catalysts include metal soaps, metal oxides, metal chelates such as chromium acetylacetonate, metal salts of thiols or dithiocarbamic acids, organometallic compounds such as dibutyltin dilaurate and phenyl mercury acetate and basic nitrogen-containing compounds, preferably amines and substituted amines such as triethanol amine. The majority of the examples in this patent employ dibutyltin dilaurate as the catalyst. While this compound and other organotin compounds yield acceptable cured polyorganosiloxane elastomers which retain their initial properties virtually indefinitely at ambient temperatures, polyorganosiloxanes prepared using organotin compounds wherein tin is in a tetravalent state are not stable when exposed to temperatures above about 150.degree. C. At these temperatures the organotin compound catalyzes an irreversible depolymerization to low molecular weight liquid or semi-solid products. Organotin compounds therefore cannot be employed if the cured elastomers are to be exposed to the operating temperatures of high power electronic equipment, which often exceed 150.degree. C.
It is possible to avoid the problem of depolymerization or "reversion" at elevated temperatures by employing a stannous compound such as stannous-2-ethylhexoate in place of an organotin compound. While stannous compounds as a class may be useful for certain applications the catalytic activity of these compounds is so high that even at minimum effective concentrations the working or "pot" life of a catalyzed composition is extremely short and may be as short as five minutes. For certain applications, particularly when thicker sections are desired, stannous compounds as a class cannot be employed. The uppermost layer may cure rapidly to form an impenetrable skin while the lower layers remain uncured or only partially cured.
Surprisingly it has now been found that stannous salts of certain ethylenically unsaturated hydroxycarboxylic acids containing from 10 to about 20 carbon atoms are unique among stannous compounds in that the working life of a typical uncured polysiloxane composition is one hour or more. In addition, the properties of the cured product, including hardness, are equivalent or superior to those obtained using conventional catalysts such as amines and tin compounds.